Port 25 Blocking

Many ISPs are blocking what is called "Port 25" which is the
port used to send e-mail. They are doing this to cut down on the amount
of spam that is sent from their networks.

All e-mail sent via the Internet is routed through the port 25, the
channel used for communication between an e-mail client and an e-mail
server. Even though port 25 blocking will probably become an industry
standard, however, the filter can create problems for e-mail servers and
block legitimate e-mail as well as spam.

Port 25 blocking allows ISPs to block spam sent out through their networks,
but it tends to punish the innocent that have a need to send through e-mail
servers other than those belonging to their ISP. The ISPs that block port
25 require their SMTP server to be used instead of the remote SMTP server
or a SMTP server running on your computer.

How the port 25 is used

All e-mail sent via the Internet is routed through port 25. When an
e-mail server that runs on your computer delivers messages, it always
uses port 25 to transmit data to remote e-mail servers. Therefore, if
your ISP is blocking the port, your messages will not get through. There
are two different ways the port 25 is being used by PostCast Server:

Incoming Connections

PostCast Server uses port 25 to accept incoming connections from e-mail
clients. You can freely change that value in both server and client program
and everything will continue to work because all TCP/IP connections are
directed to your computer. Unless you block connections to your computer,
the program will accept messages using any port number you specify (1-65535).

Outgoing Connections

PostCast Server also uses the port 25 for sending. It connects to remote
servers and delivers the messages from the Outbox folder. Exactly the
same rules apply except that every remote server expects the connection
ONLY on port #25. This is the standard port number and while you can change
the port number in the program to allow clients to send the messages internally,
the remote servers always use port 25. If your ISP blocks remote connections
to port 25, you cannot send any messages. PostCast Server will not be
able to connect to the remote servers.

ISPs that block Port 25

This list contains some of the major ISPs that block port 25 on their
servers:

Use socks proxy servers

You can send e-mail using the socks proxy access to a computer on the
Internet. This feature enables you to relay e-mail through other servers.
When the message is sent using a third-party socks proxy, your IP address
does not appear as the source of the message.

The best solution is to connect to your ISP's socks proxy directly if
it is provided by the ISP. Some ISPs offer access to their socks proxy
server. See if your ISP provides socks proxy access in the support section
on their web site. If they do, you can use their socks proxy server address
to configure the program to send messages. Their server's (non-dynamic)
IP address will be the source of your outgoing messages instead of your
dynamic IP address assigned to your computer
at the moment your Internet connection is established.

Use backup SMTP servers

You can specify one or more backup SMTP servers and instruct the program
to forward all messages to them. This is not a complete solution because
the program will still be unable to send messages from your computer.
For more information, see SMTP Gateways.

If you do not need to send messages

If you only want to receive messages sent to the server you can use
the "Mail Reflector" service offered by no-ip.com. This service
enables them to be the primary e-mail exchanger for your domain. When
e-mail destined for your domain arrives at their servers, they forward
it on to your inbound e-mail server, which can be on a different (and
unblocked) port of your choosing. Price is $39.95 per Year. See this web
page for more information: http://www.no-ip.com/services/mail/reflector